Audio equipment which is prone to theft, incorporates an anti-theft system which uses a secret identification code for prevention of theft. Such an anti-theft system requires the purchaser of the audio equipment to manually enter a specific identification code, which the purchaser will remember, by using, for example, numeric keys which are on the equipment. The entered identification code is compared with a code stored in the equipment, and if the entered code is correct, the equipment is activated.
Generally, in prior art security methods for automotive audio equipment, a user identification code of several digits is manually entered by various numeric keys on the front panel of the equipment, and if the identification code is correct, as a result of a comparison with a code stored in the equipment, the equipment is activated.
Consider a situation where audio equipment is stolen from a vehicle. The equipment is first removed from the first vehicle and is then installed for use in another car. That is, the equipment is once removed from a battery and connected to another battery. The condition of battery connection, that is, a change in the condition of battery connections of the two vehicles, is detected as a variation in power supply and when this condition is detected, the above security method prevents activation of the equipment and the equipment remains inactive until the correct identification code is manually entered. Therefore, if the equipment is stolen from the vehicle, the equipment cannot be operated by persons other than the user of the equipment who knows the secret identification code of the equipment. An indication or warning to the effect that such an anti-theft feature is incorporated in the audio equipment is visibly attached to the vehicle or the equipment so as to deter potential thieves. Such an indication or warning attached to the vehicle or the audio equipment, implying that it is futile to steal the equipment, discourages potential burglars that may be contemplating stealing the equipment, thus preventing the equipment from being stolen.
However, not all audio equipment is provided with a large number of numeric keys, and the fewer the number of numeric keys, the fewer the number of possible combinations that can be used to code an identification number, resulting in a lower security level. One way to avoid this would be to use keys other than numeric keys to create a secret identification code, but this would increase the complexity of operation. Furthermore, it is more difficult for the user to remember the identification code if non-numeric keys are used.
In view of the foregoing, a security system for audio equipment which uses a recording medium, such as a compact disc, as a key has been proposed as disclosed in Japanese Patent Application No. 2-333681 (Japanese Patent Unexamined Publication JPA 4-205965(1992)) in which a security method for audio equipment uses recorded voices on the recording medium. This security method for audio equipment is characterized in that the information recorded on the recording medium is compared with information stored in an internal memory of the audio equipment, and the audio equipment is activated depending on the result of the comparison. Since this method requires only that the audio equipment recognizes the recording medium that is to be used for releasing a security operation, it is easier for a user to remember the recording medium than a secret identification code. Furthermore, since an enormous number of recording media are available for use, its security level is very high.
However, when a recordable medium is used as the recording medium, there is a possibility that the information recorded on the recording medium may be rewritten and may no longer match the information stored in the audio equipment, thus making it impossible to release the security operation. For example, in an audio system that uses a photomagnetic disc called a mini disc which has recently come into widespread use, a rewritable disc may be used, and the above problem occurs in a security system for a playback apparatus or a playback/recording apparatus which uses such a mini disc.
It is accordingly an object of the present invention to provide a security system for audio equipment which allows users to easily release the security operation without compromising a security level, and which is prevented from being put in unlockable condition by careless handling.